Johnson, the former top lawyer at the Pentagon, told the Senate Judiciary Committee that his agency’s lawyers will review federal policies, which he said were based on a Supreme Court ruling, that require the government to release criminal immigrants if the U.S. can’t send them to their home countries within six months.

Johnson said he wants to know whether the government can hold them in immigration jails as threats to national security or public safety. Johnson said he wanted to be sure that the government wasn’t construing the Supreme Court decision too narrowly.

Johnson also said he wants U.S. officials with higher seniority to review and approve such releases in sensitive cases.

Last month, the Obama administration acknowledged it has released more than 36,000 criminal immigrants living in the country illegally, including those accounting for 193 homicides and 426 sexual assaults.

In many cases — but not all of them — the U.S. was required to release the immigrants while their deportation cases are pending. The immigrants nearly all still face deportation and are required to check in with immigration authorities while their deportation cases are pending.

Johnson previously told a House panel that he wanted a “deep understanding” of the circumstances that led to those releases. He told senators at the hearing that an internal review of those releases in ongoing.

Grassley complained anew during the hearing that the administration was releasing criminals to the community with little or no oversight and asked Johnson to provide a fuller accounting to his office.

Republican lawmakers have argued that the Obama administration isn’t properly enforcing immigration laws. Several of them, including Grassley, have also blamed the administration’s enforcement practices for a recent spike in the number of child immigrants from Central America caught trying to cross the Mexican border without their parents.